Bond Enthalpies

Bonds are broken and new bonds are formed in chemical reactions. Energy changes take place in breaking some bonds of the reactants and in forming new bonds of the products. So, the energy changes in a chemical reaction are a result of energy changes in breaking and forming of bonds.

For a diatomic molecule like H₂ (g), bond dissociation energy is defined as the enthalpy change of the reaction in which one mole of the gaseous molecules are dissociated into gaseous atoms.

H₂ (g) → H (g) + H (g) ; Δ_rH = 435 kJ/mol

Bond enthalpy is defined as the average amount of enthalpy change involved in the dissociation of one mole of bonds present in different gaseous compounds.